1. Field of the Invention
The present invention relates to a structure of a stepping motor and a method for driving a stepping motor by switching the number of excitation phases between one and two.
2. Related Background Art
In a stepping motor, a method for driving the stepping motor by switching the number of excitation phases by a drive circuit has been adopted.
In a prior art method for driving a stepping motor by switching the number of excitation phases, the generated torque varies with the number of excitation phases so that vibration and noise are generated and non-uniform rotation is caused.
In the structure of a prior art stepping motor, plural phases (normally two phases) of a stator unit comprising a yoke and a coil are axially arranged around a rotor having magnetic poles.
In such a stepping motor, it is common to axially arrange two stator units each comprising an inner yoke and an outer yoke having a plurality of magnetic teeth circumferentially formed at a constant pitch around a rotor having magnetic poles, and a coil mounted between the yokes.
In such a prior art stepping motor, the inner and outer yokes of each of the stator units are constructed such that the circumferential spacing between the magnetic teeth thereof is uniform.
In such a structure however, a variation torque or cogging torque for each step angle increases due to magnetic interference from the other adjacent stator unit. This prevents a constant speed of rotation.
Further, because of the large cogging torque, the output torque is apt to vary.